Arrangement for Synchronizing Access Points in WLAN Using Direct-Sequence Spread Spectrum Signaling

ABSTRACT

An arrangement for providing synchronization between a number of overlapping area access points within a wireless LAN utilizes an Ethernet cable connection to transmit synchronization signals from a centralized hub/switch to each of the access points connected to the cable. The synchronization signal takes the form of a direct sequence spread spectrum (DS-SS) signal. At each access point, a filter is used to recover the synchronization signal, allowing for the group of access points to share a common beacon signal and allowing frequency re-use among the access points.

CLAIM OF PRIORITY

This application is a continuation of and claims priority from U.S.patent application Ser. No. 12/636,866, filed on Dec. 14, 2009, which isa continuation of U.S. patent application Ser. No. 11/505,553 filed onAug. 16, 2006, now issued as U.S. Pat. No. 7,656,855, which claims thebenefit of Provisional Application No. 60/796,177, filed Apr. 29, 2006,the contents of each of which are expressly incorporated herein byreference in their entirety.

FIELD OF DISCLOSURE

The present invention relates to an arrangement for synchronizingtransmission between a plurality of access points (APs) in a wirelesslocal area network (WLAN) and, more particularly, to an arrangement thatutilizes direct-sequence spread spectrum (DS-SS) signaling.

BACKGROUND

A wireless local area network (WLAN) is generally implemented to providelocal connectivity between a wired network and one or more mobilecomputing devices. In a typical wireless network, all of the computingdevices within the network broadcast their information to one anotherusing radio frequency (RF) communications. WLANs are based on the IEEE802.11 standard, which designates a wireless-Ethernet specificationusing a variety of modulation techniques at frequencies in the 2.4 GHzand 5 GHz license-free frequency bands, although other bands might beused with the same basic framework.

The IEEE 802.11 standard (generally referred to as “Wireless Fidelity”,or Wi-Fi) enables wireless communications with throughput rates up to 54Mbps. Wi-Fi is essentially a certification that a manufacturer's productis interoperable with the IEEE 802.11 standard. For example, equipmentcarrying the “Wi-Fi” logo is certified to be interoperable with othercards that operate in peer-to-peer mode, but Wi-Fi usually incorporatesat least one access point, or edge device. Most access points have anintegrated Ethernet controller to connect to an existing wired-Ethernetnetwork. A Wi-Fi wireless transceiver connects users via the accesspoint to the rest of the LAN. The majority of Wi-Fi wirelesstransceivers available are in Personal Computer Memory CardInternational Association (PCMCIA) card form, particularly for laptop,palmtop, and other portable computers. However, Wi-Fi transceivers canalso be implemented through an Industry Standard Architecture (ISA) slotor Peripheral Component Interconnect (PCI) slot in a desktop computer,mini-PCI cards embedded in a laptop computer, a Universal Serial Bus(USB), or can be fully integrated within a handheld device.

Modern WLANs composed of more than one access point with overlappedcells require a frequency re-use pattern to minimize interference.However, available frequencies may be too few to allow for a sufficientamount of co-channel interference reduction. Recently, newquality-of-service (QoS) protocols allow for APs to share “time” as wellas “frequency”. However, synchronization between APs is required forsuch time division multiplexing schemes. One prior art arrangement forproviding synchronization between access points is discussed in U.S.Pat. No. 6,917,804, issued to T. Takayama et al. on Jul. 12, 2005. Inthis case, one access point is defined as the “master” access point andperiodically transmits a “master beacon” to all remaining access pointsin the WLAN. Such an arrangement, however requires each access point tobe equipped with additional receiving and synchronization components.

A need remains, therefore, for an arrangement that allows forsynchronization of access points with little additional overhead, whilemaintaining the reliability of a wired connection.

Other and further aspects of the present disclosure will become apparentduring the course of the following discussion and by reference to theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary prior art wireless LAN arrangementincluding a plurality of overlapping coverage access points (APs);

FIG. 2 illustrates an exemplary prior art Power over Ethernet (PoE)arrangement for providing both data and low power signals from a powersourcing equipment (PSE) module to a powered device (PD) end station;

FIG. 3 contains a diagram of an exemplary embodiment of the presentinvention, where the PoE Ethernet cable connection is used to providedirect-sequence spread spectrum (DS-SS) synchronization signals from ahub connection to an exemplary access point over a “spare” cable pair;

FIG. 4 illustrates an exemplary embodiment of a power separation filterutilized to combine a power signal (I) and DS-SS sync signal (S) ontothe same cable pair within a CAT5 Ethernet cable;

FIG. 5 illustrates an alternative embodiment of the present inventionwhere the pair of cables carrying the data signal are also used to carrythe synchronization signal;

FIG. 6 illustrates, in simplified block diagram form, a plurality ofaccess points coupled to a single hub location via an Ethernet cablethat supports the transmission of the same synchronization signal toeach access point, thus allowing for frequency re-use in accordance withthe present invention;

FIG. 7 illustrates an exemplary synchronization generator module for usein generating an exemplary DS-SS synchronization signal to betransmitted over the Ethernet cable; and

FIG. 8 illustrates an exemplary demodulator for use in an AP to recoverthe various synchronization pulses from the propagating synchronizationsignal.

DETAILED DESCRIPTION

The need remaining in the prior art is addressed by the presentdisclosure, which relates to an arrangement for synchronizingtransmission between a plurality of access points (APs) in a wirelesslocal area network (WLAN) and, more particularly, to an arrangement thatutilizes direct-sequence spread spectrum (DS-SS) signaling. The presentdisclosure is based on leveraging the communication capability providedby the emerging Power over Ethernet (PoE) technology to “piggyback”synchronization signals onto the power signals, thereby distributing thesynchronization signals to each of the APs within the wireless LANwithout requiring additional overhead, and allowing for greaterfrequency re-use between adjacent, overlapping cells.

FIG. 1 is a diagram of a conventional prior art wireless LAN system 10comprising a plurality of access points (APs) 12 coupled to acommunication switch (or hub) 14 via a conventional Ethernet CATS cable16. The radio frequency coverage for each AP 12 is illustrated as ashaded cell region 18, where the overlap between adjacent cells 18 isillustrated as cross-hatched areas 20. As mentioned above, anoverlapping coverage condition may produce too much interference forsome wireless devices in overlap regions 20, illustrated as wirelessdevices 22 in FIG. 1. That is, unless there is a frequency re-use plan,or a radio resource time-sharing arrangement within wireless LAN 10,devices 22 will experience transmission errors.

FIG. 2 illustrates a conventional prior art PoE arrangement that isutilized to provide power signal input over Ethernet cable 16 between apower sourcing equipment (PSE) module 30 and a powered device (PD) 32.While the intricacies of the power distribution are not germane to thesubject matter of the present invention, an overall understanding of thePoE system is helpful. Referring to FIG. 2, Ethernet cable 16 comprisesin this embodiment a CATS cable which includes four twisted pairs ofcopper signal paths. A first pair (designated 16-1) is used to providedata and power signals from PSE 30 to PD 32. A second pair (designated16-2) is used to provide the return data signals from PD 32 to PSE 30.The remaining pairs (16-3 and 16-4) have previously been reserved as“spare” communication lines in the event of a failure of either primarypair. Alternatively, and particularly for high speed applications, cablepairs 16-3 and 16-4 may be used as a second data line. When implementingPoE, the power signal is transmitted over data pair 16-1, 16-2 (or“spare” pair 16-3, 16-4) in a “common mode” powering arrangement. If thespare pairs are used for powering, the individual wires in a pair areshorted together at both ends, allowing both conductors to share thecurrent being supplied.

With this understanding of the capability of distributing low voltagepower signals over Ethernet cable, the subject matter of the presentinvention may be explained as follows. Referring to FIG. 3, an exemplarywireless LAN synchronization arrangement for overlapping APs isillustrated as comprising a hub 14 and an exemplary AP 12 connected viaEthernet cable 16. For the sake of clarity, only a single AP 12 is shownin this view. In accordance with the present invention, hub 14 comprisesa PSE 30, similar to the arrangement of FIG. 2. Hub 14 further comprisesan Ethernet line driver 31 for supplying an input data signal (E) topair 16-1 of cable 16, and an Ethernet line receiver 33 for recoveringreceived data from cable pair 16-2 of CATS cable 16.

In accordance with the present invention, hub 14 further comprises asynchronization generator module 40 that is utilized to generate adirect-sequence spread spectrum (DS-SS) synchronization signal S that iscoupled through a first power separation filter 42 and transmitted to AP12 over cable 16. It is to be understood that sync signal S may begenerated and transmitted to AP 12 whether or not power I is also beingtransmitted to AP 12 via PoE over cable pair 16-3. Advantageously, thecreation of a DS-SS signal for sync signal S allows for a relatively lowpower, broadband signal to be formed that will not interfere with thedata also being transmitted over cable pair 16-1 (or with any powersignal than may be present along cable pair 16-3).

At the receive side within AP 12, a second power separation filter 44 isused to separate sync signal S from any power signal I that may bepresent along pair 16-3. Synchronization signal S is thereafter appliedas an input to a DS-SS synchronization demodulator 46 to recover thespecific sync signal S, as identified below, that is used to maintainoverall synchronization between the plurality of separate access points.The power signal I (if present) is then applied as an input to a powersinking device (PD) 32, as shown in FIG. 3.

In a preferred embodiment of the synchronizing method discussed hereinand as particularly illustrated in FIG. 3, spare pairs 16-3, 16-4 ofCAT5 cable 16 are employed to transmit the synchronization signals (andpower signals) between hub 14 and a plurality of APs 12. In this case,first power-separation filter 42 is interposed between PSE 30,synchronization generator module 40, and CAT5 cable 16 at hub location14. It is to be understood that if a plurality of CAT5 cables areassociated with hub 14, a single power supply, power separation filter,and synchronizing generator can be shared among the plurality of cablesto provide the desired synchronization signal.

The function of first power separation filter 42 is to prevent thehigh-frequency DS-SS sync signal S generated by module 40 from beingattenuated by the low resistance of the power supply/powering sourcingdevice 30, thus more efficiently coupling the sync signal S onto sparecable pair 16-3. FIG. 4 illustrates an exemplary arrangement of firstpower separation filter 42, sync generator module 40 and cable pairs16-3 and 16-4. An LC circuit arrangement 43 is used to maintainseparation between the DC power signals I_(dc)) and the relatively highfrequency sync signal S. In particular, as a result of itshigh-frequency content, sync signal S propagates through the cable,which behaves as a balanced transmission line, in a relatively unimpededfashion. This minimizes coupling into the data pairs and easescompliance with FCC emissions rules. Second power separation filter 44within each AP 12 is similar in form to first power separation filter 42and functions to isolate sync signal S at the output thereof.

In an alternative embodiment of the present invention, as illustrated inFIG. 5, sync signal S is directly applied to cables pairs 16-1 and 16-2in combination with data signal E. Since DS-SS sync signal S isnoise-like and spread so that its power is distributed over a widefrequency range wide, there will be a minimal amount of interferencewith Ethernet data transmission (particularly the carrier-sensefunction). However, this alternative embodiment requires that thesynchronization and data signals be combined before being fed to aline-coupling transformer at the hub. Additionally, a power signal I mayalso be transmitted over cable pair 16-1. As shown, power signal I iscoupled through a center-tapped transformer 34 onto data pair 16-1 alongwith the DS-SS signal and the data signal. Upon reaching AP 12, the lowvoltage DC current I is returned via another center-tapped transformer36, with the current passing through PD 32 as shown. The DS-SS syncsignal S is removed from the Ethernet data signal E by DS-SSsynchronization demodulator 46, in a manner similar to that discussedabove in association with the embodiment of FIG. 3. Aside from smalldegradation of the Ethernet signal-to-noise ratio, this alternativearchitecture also complicates mid-span powering since the pair of cablesmust be broken to impress the synchronization signal via metallic(rather than common mode) current while retaining line balance. This isbest accomplished by turning the mid-span powering tap into an Ethernetrepeater.

FIG. 6 illustrates, in a simplified overview, the arrangement of thepresent invention where sync signal S as created within sync generatormodule 40 of hub 14 is thereafter transmitted along Ethernet cable 16and received by each AP 12 connected to cable 16. As long as each AP 12includes the same demodulator 46, each AP 12 will recover the same syncsignal and be able to maintain synchronous communication within thewireless LAN formed by the plurality of APs 12. In particular, syncsignal S is used to align the superframes (beacons) for each AP 12 inthe network, allowing them to time-share the same frequency channelwithout causing interference. The synchronization also facilitates powersaving within the wireless client devices and provides for improvedhand-off of client devices between APs.

FIG. 7 illustrates an exemplary synchronization generator module 40formed in accordance with the present invention. In a preferredembodiment, a pair of independent pseudo noise (PN) sequences areutilized to form the DS-SS sync signal S. A first PN sequence, denotedPN-1, is used to generate a precision phase signal that is utilized asthe “pilot” tone for the other sync signals. PN-1 may comprise eitherall 0's or all 1's for fast acquisition. A time mark signal TM thatfacilitates the ability to provide the superframe (beacon) timing amongthe APs (as mentioned above) and “time of day” (ToD) coding signal arecombined within a multiplexer 50 and thereafter spread by a second PNsequence (PN-2), for example, as a real-time clock facility for AP 12,providing ranging, delay measurements and the like. Sequence PN-2 is ofthe same length as PN-1, but comprises an equal number of 0's and 1's.Alternatively, a longer PN sequence may be used for PN-2 so as to makethe signal more noise-like.

As shown in FIG. 7, each of these PN sequences is applied as a separateinput to a summer 54, where Ethernet data signal E is applied as a thirdinput to summer 54. Summer 54 adds the synchronization information(pilot, ToD, TM) to the already-existing Ethernet data signal E (similarto adding white noise to a signal) and then transmits the signal viaCATS cable 16 to the remote APs. It is to be understood that in variousother embodiments, perhaps only PN-1 is utilized, or a combination ofPN-1 and PN-2, or any other appropriate combination of one or more PNsequences that supply sync pulses to the plurality of separate APs. Theresultant DS-SS sync signal S is generally in the form of a noise-likespectra, low enough to provide negligible crosstalk with the data and/orpower signals propagating along cable 16.

FIG. 8 illustrates an exemplary demodulator 46 used within each AP 12 toextract the various PN sequences from the received signal travelingalong cable 16. As shown, the received signal is demodulated using thesame baseband signal B and a relatively straightforward filteringarrangement is then used to separate the various PN sequences from thesignal. Thus, as long as each AP 12 recovers the same sync pulses(pilot, TM, ToD), the various units will be able to provide time sharingof the frequency spectrum without the interference problems prevalent inthe prior art.

While the present invention has been described in relation to aparticular preferred embodiment, many variations, equivalents,modifications and other uses will become apparent to those skilled inthe art. Thus, the present invention is intended to be limited only bythe scope of the claims appended hereto.

What is claimed is:
 1. A method comprising: generating a direct sequencespread spectrum synchronization signal at a network hub from acombination of a baseband data signal and a plurality of pseudo noisesequences including timing information for providing synchronization;coupling the direct sequence spread spectrum synchronization signal ontoat least one Ethernet cable, wherein the at least one Ethernet cablecouples the network hub to a plurality of wireless access points of awireless local area network; and simultaneously transmitting the directsequence spread spectrum synchronization signal from the network hub tothe plurality of wireless access points over the at least one Ethernetcable.
 2. The method of claim 1, wherein the plurality of pseudo noisesequences includes a first pseudo noise sequence defining a precisionphase signal, the precision phase signal utilized as a pilot tone tomaintain synchronization between the plurality of wireless accesspoints.
 3. The method of claim 2, wherein the plurality of pseudo noisesequences further includes a second pseudo noise sequence defining atime mark signal, the time mark signal utilized as a superframe/beaconsynchronization signal between the plurality of wireless access points.4. The method of claim 3, wherein the plurality of pseudo noisesequences further includes a third pseudo noise sequence defining atime-of-day signal, the time-of-day signal usable within each accesspoint of the plurality of wireless access points to provide a real-timeclock signal.
 5. The method of claim 1, further comprising:demodulating, at each wireless access point of the plurality of wirelessaccess points, the direct sequence spread spectrum synchronizationsignal; and extracting the timing information therefrom.
 6. The methodof claim 1, wherein the direct sequence spread spectrum synchronizationsignal is generated using at least two independent pseudo noisesequences.
 7. The method of claim 1, further comprising transmitting adata signal from the network hub to the plurality of wireless accesspoints over the at least one Ethernet cable simultaneously withtransmitting the direct sequence spread spectrum synchronization signaland the power signal.
 8. A network hub device comprising: a powersourcing device responsive to a power supply to generate a power signal;a direct sequence spread spectrum synchronization generator configuredto generate a direct sequence spread spectrum synchronization signalbased on a baseband data signal and a plurality of pseudo noisesequences including timing information; and a circuit coupled to thepower sourcing device and to the direct sequence spread spectrumsynchronization generator, wherein the circuit is configured to send thepower signal and the direct sequence spread spectrum synchronizationsignal to a plurality of wireless access points of a wireless local areanetwork via at least one Ethernet cable.
 9. The network hub device ofclaim 8, further comprising an Ethernet line driver configured to send adata signal via the at least one Ethernet cable to the plurality ofwireless access points concurrently with the power signal and the directsequence spread spectrum synchronization signal.
 10. The network hubdevice of claim 8, wherein the circuit includes a power separationfilter.
 11. The network hub device of claim 8, wherein the directsequence spread spectrum synchronization signal is generated using atleast two independent pseudo noise sequences.
 12. The network hub deviceof claim 8, wherein the direct sequence spread spectrum synchronizationsignal includes a precision phase signal, wherein the precision phasesignal provides a pilot tone to maintain synchronization between theplurality of wireless access points.
 13. The network hub device of claim8, wherein the direct sequence spread spectrum synchronization signalincludes a time mark signal, wherein the time mark signal provides asuperframe/beacon synchronization signal between the plurality of accesspoints.
 14. The network hub device of claim 8, wherein the directsequence spread spectrum synchronization signal includes a time-of-daysignal, wherein the time-of-day signal is usable within each wirelessaccess point of the plurality of wireless access points to provide aclock signal.
 15. An access point device comprising: a circuitconfigured to receive a signal from a network hub via at least oneEthernet cable, the signal including a power signal and a directsequence spread spectrum synchronization signal, wherein the directsequence spread spectrum synchronization signal includes a baseband datasignal and a plurality of pseudo noise sequences; a power sinking devicecoupled to the circuit, wherein the power sinking device is powered bythe power signal; and a direct sequence spread spectrum synchronizationdecoder coupled to the circuit, the direct sequence spread spectrumsynchronization decoder configured to decode the direct sequence spreadspectrum synchronization signal to determine timing information and tosynchronize operation of the access point device with one or more otheraccess point devices coupled to the network hub using the timinginformation.
 16. The access point device of claim 15, wherein the directsequence spread spectrum synchronization signal is generated using atleast two independent pseudo noise sequences.
 17. The access pointdevice of claim 15, wherein the direct sequence spread spectrumsynchronization signal includes a precision phase signal, wherein theprecision phase signal provides a pilot tone to maintain synchronizationof the access point device and the one or more other access pointdevices coupled to the network hub.
 18. The access point device of claim15, wherein the direct sequence spread spectrum synchronization signalincludes a time mark signal, wherein the time mark signal is used toalign superframes or beacons of the access point device with superframesor beacons of the one or more other access point devices coupled to thenetwork hub.
 19. The access point device of claim 15, wherein the directsequence spread spectrum synchronization signal includes a time-of-daysignal, wherein the time-of-day signal is used to provide a real-timeclock signal.
 20. The access point device of claim 15, furthercomprising an Ethernet line receiver operable to receive a data signalvia the at least one Ethernet cable concurrently with receipt of thepower signal and the direct sequence spread spectrum synchronizationsignal.